We undertook a study to measure the difference in work of breathing, as reflected by oxygen uptake (VO2) and carbon dioxide production (VCO2), between that on a continuous-flow CPAP system (Emerson IMV) and that on a demand-flow CPAP system (Bennett MA-2). A specially designed valve permitted the isolation and collection of expired gases from the Emerson IMV/CPAP ventilator. Ten patients receiving CPAP therapy were selected for study. Each patient was alternately placed on the Emerson CPAP system and on the MA-2 CPAP system; the order was determined randomly. For each patient the fraction of inspired oxygen (FIO2) and the end-expiratory pressure were kept the same. The inspired and expired gases were collected and analyzed by the Beckman Metabolic Measurement Cart (MMC). Analyses of VO2, VCO2, respiratory quotient, minute ventilation, tidal volume (VT), respiratory rate, and FIO2 were made serially at approximately 45-second intervals for 8 minutes. A comparison between the Emerson continuous-flow CPAP system and the MA-2 demand-flow CPAP system revealed that with the MA-2 the mean values for VO2, VCO2, and VT were, respectively, 16.3%, 12.6%, and 6.4% higher, a statistically significant difference. We believe that this difference was probably due to the additional effort required of the patient on the MA-2 to open and close the demand valve.